I’m trying to get in a habit of starting my mornings off with coffee and a Leetcode problem before work. Ease my way into the day and get the brain juices flowing. Let’s jump in!
Add Two Numbers
You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.
You may assume the two numbers do not contain any leading zero, except the number 0 itself.
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4) Output: 7 -> 0 -> 8 Explanation: 342 + 465 = 807.
This one took me a few swings, mainly because I didn’t read that the return value was supposed to be a linked list. I had implemented the logic to sum the linked lists properly and then realized I’d have to iterate over the resulting integer to build a list. Bleh. So I started over so I could implement it in a single iteration of
The solution works as follows:
Continue looping while either of the nodes has a
Sum digits from each list. Use zero if we’ve exhausted one of the lists. Track a “carry” if sum is greater than 10. Result list should only store
sum % 10 so it only stores a single digit.
If we’re at the end of both lists, check to see if we still have a “carry”. If so, need to append another node to result list with the value of
The following corner cases are what I missed in my initial solution:
- The final “carry” when both lists were exhausted.
I feel like the tracking of the head node relative to the result list moving pointer code is messy.
# Definition for singly-linked list. # class ListNode: # def __init__(self, x): # self.val = x # self.next = None class Solution: def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode: carry = 0 head = l3 = ListNode(None) while True: if not l1 and not l2: if carry: l3.next = ListNode(carry) break l1_val = l1.val if l1 else 0 l2_val = l2.val if l2 else 0 value = l1_val + l2_val + carry digit = value % 10 if l3.val is None: l3.val = digit head = l3 else: l3.next = ListNode(digit) l3 = l3.next carry = 1 if value >= 10 else 0 l1 = l1.next if l1 else None l2 = l2.next if l2 else None return head
Runtime: 68 ms, faster than 77.41% of Python3 online submissions for Add Two Numbers. Memory Usage: 13.8 MB, less than 5.67% of Python3 online submissions for Add Two Numbers.